PVC-based, polyolefin-based or rubber-based surface coverings are widely used to decorate buildings or houses surfaces as they are more pleasant, more aesthetic and more comfortable than painted surfaces. However, these surface coverings, especially floor coverings, are generally produced in tiles or panels that have to be joined one to the others. For this purpose, the use of a welding rod is well known. Usually, a round cross-section thermofusible welding rod is introduced in a hot-air welding device heating the welding rod that penetrates into the space between two pieces of adjacent floor-covering elements and joins them together. Once cooled-down, the excess of welding rod is cut off, or skived, with a “U”-shaped cutter.
Welding rods are usually polymeric compositions having good adhesion to the covering elements to be joined, and allowing the welding rod to have flexibility at temperatures lower than the composition of the elements to be joined, while being easily cut off after welding. Furthermore, a welding rod has a melt temperature lower then the floor-covering elements to join, to avoid the melting of said floor-covering elements when joining them with the welding rod.
For floor coverings having a multicolour decorative pattern, it has been suggested to use multicoloured welding rods. Thus, from an aesthetic point of view, the welding rod has to emulate the aspect or the pattern of the surface covering.
Generally, for PVC-based coverings, multicoloured welding rods are PVC-based compositions and are produced by extrusion. However, extruded coloured welding rods appear as a seam of a contrasting colour (FIGS. 1 to 3), as they are unable to properly reproduce a complex multicoloured pattern, especially a three-dimensional covering pattern.
For coverings comprising a multicoloured chips-image or presenting a three-dimensional aspect, decorative welding rods incorporating multicoloured chips or granules were developed. However, for such decorative welding rods, the extrusion process was not suitable because of the conflicting requirements for the extrusion process and for the welding process of surface covering elements.
Therefore, it has been suggested to produce decorative welding rods by pressing a PVC-film, which has been calendered beforehand, in a half-round embossing mould, as described in EP 0 775 563.
WO 00/26004 describes a process in which coloured granules are spread onto a conveyor belt having grooves and said granules are then hot-pressed to melt one to the others, in order to form a sheet which is then cut into welding rods. While U.S. Pat. No. 5,635,266 describes a process in which coloured chips are deposited on a support carrier and consolidated into a sheet, which is cut into strips and press-moulded into half-round cross-section welding rods.
In addition, US2003/0072936 describes a process in which a first layer of pigmented PVC particles are deposited onto a release paper and a second layer of transparent or translucent solid PVC particles is deposited onto the pigmented particles, particles of the first and the second layer being fused with heat and pressure in a rolling press process.
However, if the welding rods of the prior art emulate the colours and the general aspect of the floor-covering elements to weld, they are not visually integrated to the pattern of the floor covering. Especially for three-dimensional coverings, the chip image of the welding rod creates an image that is different from the original covering, leading to a discontinuity in the pattern of the surface covering.
A solution to emulate the aspect of the floor-covering elements to weld can consist in producing a plurality of welding rods by hot-embossing one or two decorative surface-covering elements in a cylindrical embossing device, as describes in EP 1 619 009; However, as it is well known, a welding rod have to have properties different from those of floor-covering elements, which are usually multiple layers elements. This solution is not satisfying as such welding rods do not present the required properties, for example in terms of melt temperature, flexibility, adhesion and of cut off properties a welding rod should have. Indeed, the welding rod according to EP 1 619 009 do not succeed in an adhesive traction test due do their poor adhesion and joining properties. Furthermore, they do not have a sufficient diameter allowing a good adhesion and an easy cut off.